fix(transaction-fee): reject multi-subnet alpha fee deduction

[loom-agent]

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Release Notes

• Fixed a fee-evasion path where multi-subnet alpha fee deduction silently returned zero fee, allowing a validate-dispatch state change to skip payment.

Problem

When alpha_vec.len() != 1, the transaction fee handler returns Ok((0, 0, NetUid::ROOT)) instead of rejecting the transaction. Since validate passes with zero fee and pre_dispatch charges zero fee, a caller with a multi-subnet alpha vector can bypass payment entirely.

Root Cause

In pallets/transaction-fee/src/lib.rs, the withdraw_in_alpha implementation for TransactionFeeHandler returns a zero-fee Ok for the multi-subnet case, matching can_withdraw_in_alpha's behavior. But can_withdraw_in_alpha is informational; withdraw_in_alpha is the actual payment path and must reject what it cannot charge.

The Fix

Change the multi-subnet branch in withdraw_in_alpha from Ok((0, 0, NetUid::ROOT)) to Err(InvalidTransaction::Payment.into()), making it consistent with the validate-dispatch state-change protection.

Testing

Updated test_rejects_multi_subnet_alpha_fee_deduction to assert Err(InvalidTransaction::Payment) instead of Ok(...). The test now verifies the rejection is explicit.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works

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